Generator Size Calculator — Watts & kVA

A refrigerator that runs on 700 watts needs about three times that for a second or two every time its compressor kicks in. That start-up surge, not the running total, is what trips an undersized generator. List what you need during a power cut and this calculator adds up the running watts, finds the one appliance with the biggest surge, and rounds up to a generator size you can actually buy, in watts and in kVA.

Type appliances in yourself or add common ones from the list of typical ratings. Untick a row to see the difference it makes without deleting it. Watts never depend on the voltage, but the running current does, so pick 120, 230 or 240 V to see the amps.

Worked Example: A Home Outage List

The calculator opens with a refrigerator (700 W running, 2,200 W starting), a 1/2 hp sump pump (1,050 / 2,150 W), LED lights, a TV and router and a microwave. It reports:

• Running load: 3,100 W, which is 13.48 A at 230 V
• Biggest start-up surge: +1,500 W from the refrigerator, more than the pump’s +1,100 W
• Peak: 4,600 W, so the answer is a 5,000 W generator, 6.25 kVA at a 0.8 power factor, running at 62% load
• If both motors started in the same instant the load would touch 5,700 W, and the tool warns you to switch them on one at a time

Load Sample moves on to a window air conditioner with four ceiling fans, a workshop with a table saw and a 1 hp compressor, and an RV with its roof air conditioner.

How the Math Handles Start-Up Surge

Peak load is every running watt plus the largest single surge, because motors on the same supply almost never start at the same moment. The size needed is the higher of that peak and the running load plus your headroom, 20% by default, which keeps the generator below about 80% of its rating for hours at a time. kVA is watts divided by the power factor. Each row is checked: a starting figure below the running figure is refused with “starting watts can’t be lower than running watts. For an appliance without a motor, use the same number for both.”

When Not to Use a Watts List: Transfer Switches and Wiring

The library ratings are typical values. A nameplate or a clamp meter on your own appliance beats them, and inverter air conditioners and refrigerators start far more gently than the figures listed. The result says nothing about how to connect the generator. Feeding a house through an outlet can send power back down the line to utility workers, so a standby set needs a transfer switch or interlock fitted by an electrician. Above 60 kW the tool stops suggesting a size, because that is a commercial, usually three-phase, installation that should be sized from a measured load study.